Added 'html' as an "allowed extension"

It's unclear to me why you would want to limit files extensions, and this addition has stopped [my project](https://github.com/flovan/headstart) from working, without an easy fix (other than this PR) in sight. I hope you won't mind accepting this small change.

Side note: please publish this module to NPM so future problems can be avoided. The only way of having the latest version is by linking to the Github version, which is subject to changes at any given time.